Embodiments of the invention relate generally to electric machines having high power-density and, more particularly, to non-permanent magnet electric machines having high efficiency and low cost.
The need for high power density and high efficiency electric machines (i.e., electric motors and generators) has long been prevalent for a variety of applications, particularly for hybrid and/or electric vehicle fraction applications. Due to energy supply and environmental reasons, there has been increased motivation to produce hybrid-electric and/or electric vehicles that are both highly efficient and reliable, yet reasonably priced for the average consumer. However, the drive motor technology available for hybrid-electric and electric vehicles has generally been cost-prohibitive, thereby reducing one (or both) of consumer affordability or manufacturer profitability.
Most commercially available hybrid-electric and electric vehicles rely on internal permanent magnet (IPM) electric machines for traction applications, as IPM machines have been found to have high power density and high efficiency over a wide speed range, and are also easily packaged in front-wheel-drive vehicles. However, in order to obtain such high power density, IPM machines must use expensive sintered high energy-product magnets. Furthermore, IPM machines run at high speed (e.g., 14,000 rpm) to obtain optimum power density, and this high speed operation results in a high back electromagnetic field (EMF). Such high back EMF requires the use of high voltage inverter devices, which results in further increases in overall system costs.
IPM machines also involve intricate rotor and stator designs that are sensitive to high speed operation, thereby increasing the complexity and cost of their manufacture. For example, the stator of an IPM machine generally uses either a three-phase distributed winding or a hair-pin rectangular wire for higher slot fill. These windings are expensive to produce using conventional automatic winding machines. While special automatic winding machine stations may be developed to produce such windings, such tooling customization is also quite costly. Distributed winding coils also extend beyond the stator core on which they are wound, which may be disadvantageous for use in tight packaging situations such as those present in electric and/or hybrid-electric vehicles.
Furthermore, the rotor of IPM machines usually has one or more layers of cut-outs to enable insertion of the magnets, thereby leaving thin bridges between the magnets and the outer surface of the rotor. These thin bridges result in a weakened mechanical connection, which may be problematic due to high centrifugal forces when the rotor is operated at high speeds.
IPM machines also necessitate a small air gap between the stator and rotor (e.g., 0.02-0.03 inches) in order to achieve high power density and high efficiency. The need for a small air gap means that both the stator and rotor must be manufactured with tighter tolerances, thereby adding to the complexity and cost of their construction.
Another drawback to the use of IPM machines is the need for sintered magnets to be used in the rotor if high power-density is to be achieved. These sintered magnets cannot be bonded and must be inserted into the rotor channels un-magnetized, after which the magnets are glued and the assembled rotor is balanced. The rotor is then “dropped” into the stator, the IPM machine is assembled, and the magnets are thereafter magnetized individually using a specialized magnetizing fixture. This process of rotor construction is not easily automated, again adding to the overall expense of manufacturing IPM machines.
For at least the reasons set forth above, the high costs of manufacturing and maintaining IPM machines have limited both the commercialization of hybrid-electric and electric vehicles and the adoption of electric drive motor technology in general.
In addressing the need for more cost-efficient and low-maintenance hybrid-electric and electric technologies, much effort has been made to develop new battery and inverter technologies. However, as evidenced above, there remains a great need for improved and cost-effective drive motor technologies before hybrid-electric and electric drive technologies become fully commercially viable.
It would therefore be desirable to provide a non-permanent magnet electric machine having a high power-density, high efficiency, and relatively low cost.